The ability of infants to respond appropriately to mother, father, siblings and strangers is a measure of their social competence. Impairments of social behavior predict future psychopathology and can also increase the likelihood of abuse. As it is known that infant behavior can be a risk factor for abuse, understanding the regulation of infant social behavior and the formation of early-life social bonds is vital in the creation of successful interventions that can lead to improved emotional and physical health. An animal model of the formation and regulation of early-life social bonds would be an important asset in this work. This laboratory has developed a model with infant rats that can be applied to the formation of a social relationship to the sire or another adult male, perpetrators of a high proportion of child abuse in humans. The proposed experiments test whether the learning processes underlying the creation of infantile odor preferences are a critical component of the formation of a positive social bond with the sire. The experiments will also identify sensitive periods; whether individual recognition is an integral part of the altered social behavior; and the role of genetic relationship and/or prenatal experience in the process. Both recent social interactions and ontogenetic history can greatly alter the intensity of the separation response of rat pups, as measured by isolation-induced ultrasonic vocalizations (USV). Brief contact of an isolated pup with its dam causes both a reduction of USV rate in her presence, 'contact quieting', and also dramatically increases the pup's subsequent isolation-induced vocalization rate, 'maternal potentiation'. In dam-reared pups, contact with their unfamiliar sire produces 'behavioral inhibition', a fear-like immobility and suppression of USV. After being reared both dam and sire, isolated pups will quiet during reunion with their familiar sire, but also nose its fur and burrow under its body. The sire's removal now induces increased calling. Together, these behaviors indicate the formation of a profoundly altered social relationship. The biparental rearing-USV testing paradigm is a unique and novel model with wide applications for investigation of the neurobehavioral processes regulating formation of early-life social relationships and their later-life consequences. Studies using rats will elucidate the basic building blocks from which the more complex social interactions and attachments of humans are shaped. [unreadable] [unreadable] [unreadable]